Generate `start-link/0` in `joxa-otp-supervisor`

This changes the `gen-start-link` macro in `joxa-otp-supervisor` so that
it also generates a function `start-link/0` which does nothing more than
call `start_link/0`.
Merge pull request #49 from KlausTrainer/joxified-joxa-otp-supervisor

Generate `start-link/0` in `joxa-otp-supervisor`